FOUND PROBLEM. DISCOVERED THAT THE CAD CELL THAT 'LOOKS' AT THE FLAME WAS SLIGHTLY DIRTY, SO I WIPED IT CLEAN AND FURNACE WORKS GREAT AGAIN. I SUSPECT THAT THE SLOW ACCUMULATION OF DIRT WAS REDUCING ELECTICAL OUTPUT OF CELL TO THE POINT THAT THE RELAY BEGAN TO 'LET GO'. THE DIRT FORMED ON THE RIM OF THE CELL, NOT DIRECTLY ON THE SENSING SURFACE. MY GUESS IS THAT THE DIRT REFRACTED THE LIGHT AWAY TO THE POINT THAT THERE WASN'T ENOUGH LIGHT TO GENERATE A HIGH ENOUGH VOLTAGE TO HOLD THE RELAY CLOSED. WHY IT WORKED FINE THRU FIRST COMPLETE CYCLE IS NOW A MYSTERY.